Vertical distribution of diazotrophic bacterial community associated with temperature and oxygen gradients in a subtropical reservoir

Nitrogen-fixing microorganisms play important roles in the structure and function of aquatic ecosystems. However, the diversity and distribution of diazotrophic bacteria along the lake depth continuum are so far poorly understood. In this study, we investigated the dynamic variations of diazotrophs in a subtropical deep reservoir during the stratified period. We applied an in-depth biomolecular approach (DGGE, clone libraries, and quantitative real-time PCR) to explore the nitrogenase (nifH) gene diversity and abundance. The diazotrophic community shifted between the oxic/anoxic interface and the nifH diversity increased with depth. The Cyanobacteria, affiliated to the toxic bloom-forming Cylindrospermopsis raciborskii, were the dominant diazotrophic cluster in the surface waters, whereas diazotrophic Alphaproteobacteria were dominant in the bottom waters. The relationships between microbial and environmental factors clearly demonstrated that the temperature gradient and the oxygen concentration affect the heterogeneity of the diazotrophic community, thereby influencing the entire aquatic nitrogen cycle.